2. Peat & Settlement
2.4 Peat Investigation
Peat is a soft, fine grained soil. Traditional methods of drilling such as auger or rotary drilling function to retrieve peat samples. However, these machines are typically mounted on the back of large trucks that are heavy and hard to maneuver in remote or peaty locations. Oftentimes, the ground is too soft for access. In some cases, large wooden boards can be placed on swampy ground to allow for access, but this may not always be the case. In any event, traditional drilling methods through peat soils can be complicated and costly.
Handheld augers are an alternative method to retrieve peat samples. Although limited in depth, these do not require large machinery, and can be operated by one or two technicians or engineers. These hand-augers usually advance in either 0.5 m or 1 m intervals, and require
Deformation of Norwegian Peat Omar Berbar increasing physical difficulty with depth and time consumption when excavating deeper holes (due to the side friction of the auger and the manual labour required).
Figure 3 (L) Peat exploration near Hinton, AB, Canada. An auger drill rig recovers 3 m of peat, while positioned on a swamp. The auger rig is placed on wooden plyboards for stability.
(R) A close-up of the peat sampled. Fibres and plant material can be observed (Photos take by Omar Berbar).
For shallow surficial peat deposits, test pitting is another method of subsurface investigation.
This requires a backhoe or excavator to construct test pits. These give a more accurate depth profile compared with auguring, and allow for more uniform sampling. Test pitting is limited however by the size and weight of the excavator, which sinks in swampy ground. Large wooden boards could be used to improve mobility, but sinking an excavator is an expensive error, and not one many are willing to take a chance on. Test pitting is also limited to a maximum 5 m depth. This does generally cover usual peat deposits, but deeper peat cannot be accessed with this method.
2.4.1 Sampling
Samples retrieved from hand operated auger rigs as well as drill rigs are considered disturbed.
The investigation process disturbs the structure of the peat and therefore the samples can not
Deformation of Norwegian Peat Omar Berbar samples is a common method. This includes excavating and cutting out a block from the excavated sample, leaving the soil as undisturbed as possible. Excavation can be hand or machine operated. Sample quality affects stress history deformation parameters derived from laboratory testing and can result in an underestimation of compression (Long & Boylan, 2013).
Disturbed samples are taken from hand auger or machine drilling and used for simple testing that do not require in-situ soil structure, such as determining moisture contents.
2.4.2 Other Investigative Methods
Field vanes are a common way of assessing in-situ peat shear strengths. These apparatuses penetrate the ground surface with a multi-pronged tool and shear until the soil ruptures. Field vanes can give readings on both peak and remoulded shear strengths. However, due to the presence of fibres amongst other factors, the values can be distorted. Long et. al (2011) found that field vane results in peats are usually grossly underestimated and should be corrected (Boylan & Long, 2013).
The direct shear test is a common laboratory test used to evaluate peat shear strengths. This test requires an undisturbed block sample of peat to be sheared and give estimates for undrained shear strength.
Another investigation method is using exploratory geophysics to obtain shear wave velocity, and correlating with geotechnical parameters based on empirical and theoretical calculations (Trafford & Long, 2017). This method required some sampling to be carried out to measure moisture levels and can be an efficient way to estimate in-situ shear strengths. This method however does not consider the effects of fibers, which can sometimes be significant (Trafford
& Long, 2017).
Due to the inconsistencies and difficulty of undisturbed sampling and testing of peat a combination of different testing techniques should be used to decrease uncertainties and provide a clearer image of a peat’s strength behaviours (Zwanenburg & Erkens, 2019).
2.4.3 Exploratory Geophysics
Seismic waves are energy waves that travel through the earth’s crust and propagate through rock and soil. Naturally, these are generated from seismic activity such as earthquakes or volcanic eruptions. These waves can also be reproduced in certain geophysical methods. There
Deformation of Norwegian Peat Omar Berbar Wave propagation works in three dimensions. P-waves oscillate back and forth, with motion parallel to the direction of the wave. S-waves also oscillate back and forth, but in a different degree of freedom. The motion is perpendicular to the direction of wave propagation (Kramer, 2019).
Figure 4 Seismic wave propagation (Kramer, 2019)
Seismic geophysics works to measure ground characteristics by recreating this mechanism in scale. Vibrations from an impact at ground surface (i.e. an impact from a sledgehammer) are measured using a series of receivers, known as geophones. The impact creates shear waves that propagate through the ground. By combining the distance and time it takes from impact to recording, the shear wave velocity (Vs) can be measured. This velocity will vary depending on the medium it travels through. Certain soils will impede and slow the energy waves, while others accelerate them. Once raw data is collected, it undergoes inversion to allow for interpretation.
Deformation of Norwegian Peat Omar Berbar
Figure 5 Non-invasive measurement of shear wave velocity (L'Heureux & Long, 2017).
Exploratory geophysics are relatively non-invasive and cheaper than traditional drilling methods. Sample disturbance from traditional investigation techniques affects the quality of the laboratory data. Undisturbed sampling in peat is particularly difficult. In addition, laboratory testing does not always account for the in-situ stress levels of the soil at varying depths (L'Heureux & Long, 2017). By this token, exploratory geophysics can be useful if correlations with geotechnical parameters are established. These can a practical first-order approach in geotechnical investigations.
2.4.3.1 Correlations with Geotechnical Properties
Some work has been done to correlate shear wave velocity measurements with geotechnical parameters for Norwegian clays. In a 2017 study, L’Heureux et. al. attempt to establish a Vs
database. They identify Vs correlations with stability and strength parameters such as undrained shear strength, as well as deformation and stress history characteristics such as preconsolidation pressure. Equation 2.1 below was derived and can be used to estimate preconsolidation pressure in Norwegian clay. However, due to their uncertainties these correlations should only be used as a first-order method of soil classification and should be confirmed with laboratory testing.
(L'Heureux & Long, 2017).
𝑝𝑝′𝑐𝑐 = 0.00769𝑉𝑉𝑠𝑠2.009 (2.1) Figure 6 below plots known preconsolidation pressures calculated from laboratory testing, with shear wave velocities from the same sites. A power trendline is plotted through the data points
Deformation of Norwegian Peat Omar Berbar to give an equation. The coefficient R2 describes the efficiency of the correlation. As R2 approaches 1, the correlation increases in validity. In this case R2 is 0.8
Figure 6 Preconsolidation pressure vs. Vs, derived from clay sites around Norway (L'Heureux &
Long, 2017).